Influence of Si3N4 interface chemistry on both grain morphology and fracture resistance

Quantitative microstructural analysis was performed on Si3N4 materials doped with 5 wt% Y2O3 and 5 wt% Sc2O3 as sintering aids. Two different processing routes were utilized to achieve complete densification: (i) gas-pressure sintering of Si3N4-starting powders (SSN) and (ii) post-sintering of reaction-bonded Si3N4 (SRBSN). Apart from quantitative evaluation of grain diameter and aspect ratio of the matrix grains, the fracture toughness was determined. While the two Y2O3-doped materials (SSN, SRBSN) showed identical KIC-values, a large discrepancy in fracture toughness was found for the Sc2O3-doped Si3N4 (ΔKIC = 5 MPa√m). SEM crack propagation studies and additional TEM interface characterization showed no significant difference between these materials. Quantitative microstructure analysis, however, revealed a pronounced variation in grain diameter and aspect ratio after thermal treatment. A correlation between interface chemistry and both resulting microstructure and fracture resistance is discussed. (orig.)